In general, semiconductor-based photonic devices, such as photovoltaic cells (PV cells) and light emitting diodes (LEDs), include a junction formed between p-type and an n-type conductivity regions in a semiconductor body. In the case of PV cells, these regions generate a voltage potential and/or a current across the junction when electron-hole pairs are created in the semiconductor body in response to impinging photons on the photovoltaic cell. When a load is connected between the p-type and an n-type conductivity regions, an electric current will flow, thus producing power. In the case of LEDs, these p-type and an n-type conductivity regions generate or emit photons when sufficient voltage is applied across the junction to cause a recombination of electrons and holes.
PV cells come in two basic forms, solar photovoltaic (SPV) cells and thermovoltaic (TPV) cells. The principle operational difference between SPV cells and TPV cells relates to the energy level that photons impinging on the PV cell must have in order to generate a voltage potential and/or current. For example, SPV cells typically produce electrical energy when exposed to photons having relatively high energy levels (roughly corresponding to or occurring in the visible spectrum light). In contrast, TPV cells typically produce electrical energy when exposed to photons having relatively low energy levels (roughly corresponding to or occurring in the infrared spectrum of light).
In order to provide useful power, individual PV cells, are typically electrically and physically connected or grouped together to provide appropriate current and voltage levels. When a number of individual PV cells are electrically and physically grouped together into a single PV device the resulting PV device is often referred to as a PV cell, while the individual PV cells that make up the device are referred to as PV sub-cells. Additionally, groups of PV cells may be connected together and packaged into a scaled unit that is commonly referred to as a PV module.
PV sub-cells are typically physically connected or positioned with respect to one another in a PV cell either in strings, stacks, or in combinations of strings and/or stacks. A PV sub-cell string typically comprises two or more PV sub-cells arranged side-by-side, in line, in a horizontal string. A PV sub-cell string may be composed of a number of individual, discrete PV sub-cells connected together to form the string. Alternatively, a PV sub-cell string may be composed of a number of PV sub-cells, each of which are formed or grown as a part of a single, monolithic, crystalline structure. When each of the PV cells in a PV sub-cell string is formed or grown as a part of a single, monolithic, crystalline structure, the PV sub-cell string is typically referred to as monolithic interconnected module (MIM).
A PV sub-cell stack typically comprises two or more PV sub-cells vertically arranged one on top of the other. The PV sub-cell stack may be composed of a number of individual, discrete PV sub-cells physically bonded together to form the stack. Alternatively, the PV sub-cell stack may be composed of a number of PV sub-cells, each of which is formed together in a tandem, multi-band-gap, monolithic stack.
In addition to the various ways in which PV sub-cells are physically connected or positioned with respect to one another in a PV cell, there are also a number of ways in which PV sub-cells may be electrically connected to one another in a PV cell. PV sub-cells are typically electrically connected to one another in a PV cell either in a series type electrical connection, a parallel type electrical connection, or some combination of series and parallel type electrical connections. In a series type electrical connection, each n-type or p-type conductivity region in a PV sub-cell is connected either to an opposite n-type or p-type conductivity region in another PV sub-cell or to an output terminal or bus bar. Alternatively, in a parallel type electrical connection, each like conductivity regions in a PV sub-cell is connected to the same like conductivity region in another PV sub-cell or to an output terminal or bus bar.
The particular connection used in a PV cell to electrically connect the various PV sub-cells determines or dictates the electrical characteristics of the PV cell. For example, when each of the PV sub-cells in a PV cell are electrically connected in series, the overall voltage of the PV cell will be equal to the sum of the voltages of the PV sub-cells in the PV cell, while the overall current of the PV cell will be limited to the current value of the PV sub-cell that produces the lowest current. In contrast, when each PV sub-cells in a PV cell are electrically connected in parallel, the overall current of the in a PV cell will be equal to the sum of the individual currents of the PV sub-cells in the PV cell, while the overall voltage of the PV cell will be limited to some intermediate voltage value that is between the highest voltage value achieved by any one of the parallel connected PV sub-cells and the lowest voltage achieved by any one of the parallel connected PV sub-cells. In the case where each of the parallel connected PV sub-cells achieve a common voltage, the overall voltage achieved by the PV cell will be equal to that common voltage.
Because of the typically small voltages by each PV sub-cell in a PV sub-cell string, PV sub-cell strings are more commonly connected in series to achieve a PV cell string having a relatively high operating voltage. These high voltage PV cells offer a number of significant to advantages over low voltage PV cells, such as reduced power losses and increased overall operating voltages.
Another factor that is typically considered in designing and manufacturing PV cells is the photon absorbing capabilities of the PV-sub-cells in a PV cell. As is known, a particular PV sub-cell will absorb, and convert to electrical energy, photons with energy levels greater than a band-gap energy of the particular semiconductor material used to fabricate the PV sub-cell. When a given PV sub-cell is exposed to a radiant energy source that produces photons having a wide range of energy levels, such as the sun, only those photons having energy levels greater than or equal a band-gap energy of the given PV sub-cell will make a contribution to the electrical energy output from the cell. Conversely, those photons from the radiant energy source having energy levels less than the band-gap energy of the given PV sub-cell will make no contribution to the electrical energy output from the given PV sub-cell. As such, the energy contained in the photons having energy levels less than the band-gap energy of the given PV sub-cell is wasted.
One way in which this wasted energy can be recovered is to combine a number of PV sub-cells, each having a different band-gap energy, together in a stack of PV sub-cells. By designing the PV sub-cell stack to include PV sub-cells having different band-gap energies, photons having an energy level that is not absorbed and converted to electrical energy by one PV sub-cell in the stack may be absorbed and converted to electrical energy by another PV sub-cell in the stack.
These stacks of PV sub-cells may be composed of individual discrete PV sub-cells that are mechanically bonded together into a single stack. Unfortunately, mechanically bonding PV sub-cells together in this manner has a number of drawbacks, such as added complexity of manufacturing and concurrent increased cost to manufacture each stack of PV sub-cells. A number of MIM cells, each having a different band-gap, may also be bonded together in a single stack. Again, these mechanically bonded MIM cells share the same problems as those discussed with respect to mechanically bonded PV sub-cells.
In an alternative to physically bonding a number of PV sup-cells, a similar, but more desirable result may be achieved by producing a single monolithic tandem PV cell having multiple p-n junctions and multiple band-gaps, layered on top of the other. Layered in this manner, each set of p-n junctions may be considered to define a single, separate PV sub-cell in a vertical stack of PV sub-cells, called a PV sub-cell stack. Once arranged or fabricated in this manner, each of the individual PV sub-cells in the PV sub-cell stack are then electrically interconnected to one another in a single vertical string of serially connected PV sub-cells, where the serial connection between the PV sub-cells in the PV sub-cell stack is achieved using tunnel junctions formed between vertically adjacent PV sub-cells in the PV sub-cell stack. Unfortunately, serially interconnecting the PV sub-cells in the PV sub-cell stack in this manner limits the overall output voltage of the PV sub-cell stack to the sum of the output voltages of the PV sub-cells in the stack. As PV sub-cell stacks of this type have typically only included two PV sub-cells, the overall output voltage achieved by the PV sub-cell stack has been limited to the sum of the output voltages achieved by two PV sub-cells.
While PV sub-cell stacks of this type have proven to be highly effective, reaching efficiencies upwards of 30%, there are still a number of drawbacks and limitations associated with their manufacture and operation. For example, in order to achieve appropriate output voltages using PV sub-cell stacks, such as those just describe, it has been necessary to form physical and electrical connections between a number of these PV sub-cell stacks. Unfortunately, forming these physical and electrical connections between numbers of discrete PV sub-cell stacks adds manufacturing complexity and, thus increased manufacturing cost to the production of PV cells employing these PV sub-cell stacks. Additionally, since each of the PV sub-cells in a given PV sub-cell stack are in essence “hard-wired” in series to the other PV sub-cells in the PV sub-cell stack, the electrically interconnections that may be achieved by any PV cells employing these “hard-wired” PV-sub-cell stacks is greatly limited.
LED devices are typically constructed as either individual, discrete LED cells having a single p-n or n-p junction, or as a number of individual, discrete LED cells that are mechanically and electrically connected. The frequency of the photons (color of the light) that will be emitted by an LED is determined or dictated by the particular band-gap energy of the semiconductor material that forms the LED. For example, an LED cell that is formed of a semiconductor material having a band-gap energy of between 1.8-2.2 eV will produce a red-yellow light, an LED cell that is formed of a semiconductor material having a bang-gap energy of between 2.2-2.4 will produce a red-yellow light, and all LED cell that is formed of a semiconductor material having a bang-gap energy of between 2.4-2.6 eV will produce a blue light.
To produce an LED device that will emit light of a selected color, the device may be formed of one or more discrete LED cells, each LED being formed of a semiconductor material having an appropriate band-gap energy to produce the given color. Alternative, a number of sets of discrete LED cells, each emitting light of different color, may be situated in close proximity to one another, side-by-side, in the LED device, such that the colors produced by each of the individual LED cells will combine to produce the selected color. For example, a set of LED cells may include a red light producing LED, a blue light producing LED, and a green light producing LED. By increasing or decreasing the current supplied by each separate LED cell, and thus increasing the intensity of light produced by the LED cell, an appropriate blend of colors may be achieved by the LED device to produce a variety of perceived colors. One drawback associated with these LED devices employing three different color producing cells arranged “side-by-side,” is that the surface areas of these type of cells necessarily consume more surface area than a typical single color producing LED cell. As such, it is difficult to produce these types of cells in high densities. Another drawback associated with these types of cells in the complexity in electrical interconnects and the control circuitry that is used to control these devices. For example, it is typically necessary to have a separate pair of biasing terminals for each of the three different color producing cells.
It is against this backdrop the present invention has been developed.